Lithographic print bias/overlay target and applied metrology

ABSTRACT

A print bias target is imaged in a single layer of light-sensitive material. The print bias target is made up of a pair of concentric geometric shapes in which a plurality of target regions forms a plurality of isolated edges. Each target region is of a different image structure than each other target region immediately adjacent thereto. The positioning of a given isolated edge in the print bias target is determined relative to a corresponding isolated edge in the design image from which the print bias target was imaged. Focus setting and/or exposure setting for a lithographic system to minimize print bias may be determined using print bias information obtained from a print bias target matrix of varying exposure and focus. An imaging aberration may also be identified using print bias information from a print bias target matrix, such as lithographic astigmatism, lithographic coma and vibration. A slope of an imaging profile may also be determined from print bias information overlay may be determined by imaging a print bias target on top of a prior level image to create an overlay target, determining a center of the prior level image, determining an average center of the print bias target and determining a location of the average center relative to the prior level image center, thereby determining the overlay.

This application is a continuation of application Ser. No. 08/646,463filed May 8, 1996 abandoned which is a division of Ser. No. 08/333,110filed Nov. 1, 1994, now abandoned.

BACKGROUND OF THE INVENTION Technical Field

The present invention generally relates to lithography. Moreparticularly, the present invention relates to determining theperformance of a lithographic system.

Background Art

Lithographic processes for imaging semiconductor devices on wafersgenerally include an overlay process whereby the quality of thelithographic tool is tested by determining the quality of registrationor alignment of one lithographic level to another. In the simplest form,the overlay process consists of attempting to image an upper-level imageon top of a lower-level image such that the two images are centeredrelative to one another, and determining how well such centering isaccomplished by determining a degree of offset for the centers of theupper and lower images. The overlay measurement is accomplished by anoverlay measurement system detecting what are known as "isolated edges";that is, a boundary between two different image structures, such as asection that has been developed away and a resist section that has notbeen developed away. An isolated edge is one that can be easily detectedby the overlay measurement system, meaning that the distance between twoedges forming a line is significantly above the resolution limit of themeasurement device so that the edges are not influencing one another.

In addition to overlay, other "tests" utilizing tools other than anoverlay measurement tool are performed to determine the quality ofimaging with respect to a particular lithographic system. For example,other procedures not based on the determination of isolated edges existfor determining print bias, determining the best focus and/or bestexposure for a given lithographic system, determining imagingaberrations and determining the slope of an imaging profile. Thus, whileall of these different tests provide useful information for ensuringhigh-quality imaging, the time needed and the different tools requiredfor performing all of the tests becomes prohibitive in terms of the timeit takes to run the tests and the cost of the different equipment.Manufacturers may therefore be forced to minimize the testing ofsemiconductor devices, thus leading to a larger number of defects.

Thus, a need exists for a way to perform several different tests todetermine imaging quality of a lithographic system without having to usemany different tools and gathering many types of different data.

SUMMARY OF THE INVENTION

Briefly, the present invention satisfies the need for a way to performseveral different types of lithographic quality tests without the needfor many different types of tools and data by utilizing existing overlaymeasurement tools and the isolated edge information determined therebyto perform the various tests necessary to properly characterizelithographic image quality.

In accordance with the above, it is an object of the present inventionto decrease the number of different tools presently used to performvarious types of tests to determine image quality in a lithographicsystem.

It is another object of the present invention to decrease the differenttypes of data that are currently compiled to perform the various testsfor determining lithographic image quality.

It is a further object of the present invention to increase the numberof different tests that can feasibly be performed to determinelithographic image quality in a realistic manufacturing setting.

The present invention provides, in a first aspect, a method fordetermining print bias for a design image in a lithographic system. Themethod comprises imaging a print bias target in a single layer oflight-sensitive material, the print bias target including a plurality ofisolated edges. After imaging the print bias target, the positioning ofat least one of the plurality of isolated edges relative to acorresponding isolated edge in the design image is determined, therebydetermining the print bias. The method may further comprise a step ofadjusting the lithographic system to decrease the print bias based onthe determine print bias. The print bias target may include a pluralityof target regions, each target region having an image structuredifferent from that of each of the other target regions immediatelyadjacent thereto, the plurality of target regions forming the pluralityof isolated edges. Where the print bias target includes the plurality oftarget regions, the step of imaging may comprise imaging at least one ofthe plurality of target regions by phase shifting. In addition, the stepof imaging may comprise imaging a print bias target including aplurality of concentric geometric shapes of a single type.

In a second aspect of the present invention, a method for determining atleast one of a focus setting and exposure setting for minimizing printbias in a lithographic system is provided. The method comprises imaginga print bias target matrix from a design image in a single layer oflight-sensitive material, the print bias target matrix including aplurality of print bias targets of varying exposure and focus, eachprint bias target including a plurality of isolated edges. After imagingthe print bias target matrix, print bias for one of the plurality ofisolated edges in each of the print bias targets is determined. Amathematical model for imaging with the lithographic system is alsoderived. Once the mathematical model is derived and the print bias isdetermined, the determined print bias is correlated to the mathematicalmodel and at least one of the focus setting and exposure setting isdetermined, based on the correlated determined print bias.

In a third aspect of the present invention, a method for identifying animaging aberration in an image created in a lithographic system isprovided. The method comprises imaging a print bias target matrix from adesign image in a single layer of light-sensitive material, the printbias target matrix including a plurality of print bias targets ofvarying exposure and focus, each of the plurality of print bias targetsincluding a plurality of isolated edges in a first orientation and aplurality of isolated edges in a second orientation. After imaging theprint bias target, print bias is determined for one of the plurality ofisolated edges in the first orientation and one of the plurality ofisolated edges in the second orientation for the print bias targetmatrix. A print bias/focus plot is then created of the determined printbias against focus setting for the one isolated edge in the firstorientation and the one isolated edge in the second orientation. Basedon the print bias/focus plot, the imaging aberration is then identified.

In a fourth aspect of the present invention, a method for determining aslope of an imaging profile of an image created in a lithographic-systemis provided. The method comprises imaging a print bias target in asingle layer of light-sensitive material, the print bias targetincluding a plurality of isolated edges. After the print bias target isimaged, print bias is determined a predetermined number of times for oneof the plurality of isolated edges at each focus setting in a range offocus settings, and an average print bias for the one isolated edge iscalculated. A standard deviation for the determined print bias at eachfocus setting is determined, and a plot is created of average print biasagainst the range of focus settings for the one isolated edge. A line isthen best-fit to average print bias points in the plot corresponding toa portion of the range of focus settings having less than a maximumpredetermined standard deviation. After best fitting the line, a slopetherefor is determined.

In a fifth aspect of the present invention, a method for determiningoverlay in a lithographic system is provided. The method comprisesimaging a print bias target in a single layer of light-sensitivematerial covering a prior-level image, the print bias target including aplurality of isolated edges. A center of the prior-level image isdetermined, along with an average center for the print bias target.After the center and average center are determined, a location of theaverage center relative to the center is determined, thereby determiningthe overlay. The print bias target may comprise a pair of concentricgeometric shapes. The average center thereof being determined by findinga midpoint between corresponding sides of the concentric geometricshapes, treating the collection of midpoints on all sides of theconcentric geometric shapes as an intermediate shape, the center ofwhich is the average center for the print bias target.

In a sixth aspect of the present invention, a mask is providedcomprising a design image for imaging a print bias target in a singlelayer of light-sensitive material for determining print bias in alithographic system. The design image comprises a plurality of targetregions, each target region having an image structure different fromthat of each other of the plurality of target regions immediatelyadjacent thereto, the plurality of target regions forming a plurality ofisolated edges in the print bias target.

These, and other objects, features and advantages of this invention willbecome apparent from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art overlay target.

FIG. 2 depicts a print bias target according to the present invention.

FIG. 3 is a block diagram of an exemplary imaging system useful inperforming the various methods of the present invention.

FIG. 4 depicts the print bias target of FIG. 2 under conditions ofunderexposure.

FIG. 5 depicts the print bias target of FIG. 2 under conditions of bestexposure.

FIG. 6 depicts the print bias target of FIG. 2 under conditions ofoverexposure.

FIG. 7 is a flow diagram of a method for determining print biasaccording to the present invention.

FIG. 8 depicts the print bias target of FIG. 2 with actual and idealprofiles thereof.

FIG. 9 depicts an enlarged version of a portion of the actual profile ofthe print bias target in FIG. 8.

FIG. 10 is a plot of average print bias for isolated edges of twodifferent orientations vs. Z focus of the print bias measuring device.

FIG. 11 is a flow diagram of a method for determining a slope of animaging profile according to the present invention.

FIG. 12 depicts a matrix of print bias targets imaged at variousexposure and focus settings.

FIG. 13 is a three-dimensional plot of print bias vs. focus and exposureobtained from a print bias target matrix similar to that shown in FIG.12.

FIGS. 14 and 15 comprise a flow diagram of a method for determining bestfocus for a lithographic system according to the present invention.

FIG. 16 is a plot of print bias vs. focus setting at several differentexposures for both a horizontal and a vertical isolated edge showing noaberrations.

FIG. 17 is a plot of print bias vs. focus setting at several differentexposures for both a horizontal and a vertical isolated edge showinglithographic astigmatism.

FIG. 18 is a plot similar to that of FIGS. 16 and 17, showing thepresence of lithographic coma.

FIG. 19 is a plot similar to that of FIGS. 16, 17 and 18, showing a caseof vibration.

FIG. 20 depicts an overlay target according to the present invention.

FIG. 21 depicts an overlay target according to the present inventionwhere the prior level image is physically smaller than that of the printbias target imaged thereover.

FIG. 22 depicts the overlay target of FIG. 21 under conditions ofperfect overlay.

FIG. 23 depicts the overlay target of FIGS. 21 and 22 under a differentcondition of non-ideal overlay.

FIG. 24 is a flow diagram of a method for determining overlay in alithographic system according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a plan view of a two-level prior art overlay target 10. Target10 comprises a large box 12 created at a first lower level, and asmaller inner box 14 created at a second upper level. As shown in FIG.1, the portion of box 12 not covered by box 14 is clear, indicating thatthe light-sensitive material has been removed. The diagonal lines insidebox 14 indicate light-sensitive material that has not been removed.Assume that overlay target 10 has been created using a positive resist.Overlay is defined as the quality of registration (or alignment) of onelithographic level to another, indicating how well the mask or reticledesign is being replicated. A reticle is simply a mask where, due tomanipulation of the lithographic system, a relationship other than a 1:1relationship between the mask size and image size is created, i.e., thedesign has been reduced or enlarged.

The overlay target 10 of FIG. 1 is referred to as a box-within-a-boxtype. In the simplest terms, an overlay measurement system determineshow well inner box 14 is centered within outer box 12 in the coordinateplane the overlay target is imaged in. This determination is made bydetecting isolated edges, which are edges separating two different imagestructure regions on the target and which are not affected by otheredges that may be too close, causing interaction therebetween.

An example of an overlay measurement with respect to overlay target 10in FIG. 1 will now be given. Each of the dashed boxes around an edge,for example, dashed box 150, represents an area or detection windowwhere the overlay measurement tool will search for an isolated edge withreference to a reference point. The measurement system views overlaytarget 10 as existing in an X, Y coordinate plane. The measurementsystem first detects all of the isolated edges associated with bothouter box 12 and inner box 14. The measurement tool then calculates acoordinate pair representing the overlay determination for overlaytarget 10. The X coordinate center of outer box 12 is first found byfinding the average between the isolated edges within detection windows150 and 152. Similarly, the X coordinate center of inner box 14 is foundby determining the average of the isolated edges found within detectionwindows 154 and 156. The overlay X coordinate in then found bysubtracting the average for the X coordinate inner box from the averagefor the X coordinate of the outer box. The overlay Y coordinate value isdetermined in a similar fashion using the edges found by the measurementtool in detection windows 158, 160, 162 and 164. The focus of theoverlay measurement is a comparison of the center of the lower levelimage with the center of the upper level image.

FIG. 2 depicts a plan view of one embodiment of a print bias target 16according to the present invention. Preferably, the print bias target ofthe present invention is at least five times the resolution limit of thelithographic system used to create the print bias target. In general,print bias target 16 comprises alternating regions of two imagestructure types (i.e., polarity), referred to as a "binary" target. Thismeans that the mask or reticle image (hereinafter, referred to generallyas a "mask") producing target 16 was, in optical terms, clear in someareas and opaque in other areas. Assuming a positive resist as thelight-sensitive material, regions of target 16 that are clear correspondto regions of the mask that were-clear and in which light shown through.As one skilled in the art will appreciate, the regions of the resist inwhich target 16 was imaged that were exposed to light were developedaway, for example, by etching or the use of a solvent. The image designof print bias target 16 is of concentric geometric shapes, in this case,concentric squares.

Print bias target 16, divided into equal halves along the same axis andbroken down to its constituent target regions, comprises region 18 of afirst polarity, in this case a negative polarity. Adjacent to region 18is region 20, which is of a second polarity different than the first, inthis case a positive polarity. Adjacent to region 20 is region 22, whichcomprises a polarity different from that of region 20 and in this casethe same as that of region 18. Adjacent to region 22 is region 24, whichis of a different polarity than region 22, and in this case comprises apolarity the same as region 20. Finally, adjacent to region 24 is region26 which also comprises a different polarity than that of region 24, inthis case being the same as regions 22 and 18. As one can appreciatefrom FIG. 2, regions 20 and 26 comprise an outer box 28, and regions 22and 24 comprise an inner box 30 lying inside box 28. Region 18 ispresent so that there is a region adjacent region 20 of a polaritydifferent therefrom. Print bias target 16 is created on a singlelithographic level, unlike overlay target 10 of FIG. 1. Although printbias target 16 has been described in terms of concentric squares ofalternative image structure, it will be understood that a print biastarget of the present invention is one in which at least one isolatededge is easily detectable and is imaged in a single lithographic levelor layer.

FIG. 3 is a block diagram of an exemplary imaging system, in this caseprojection imaging system 150, and print bias measuring tool 152 usefulin performing the various methods of the present invention with respectto, for example, semiconductor wafers. Projection imaging system 150comprises an illumination source 154, mask 156, lens 158, an object forimaging 160 and an accurate X, Y, Z location controller or stepper 162.Illumination source 154 may comprise, for example, a mirror, a lamp, alight filter and a condenser lens system. Illumination source 154outputs "light" to mask 156, having a pattern thereon for replication.For example, mask 156 may include a pattern for projecting a wiringlevel of an integrated circuit under fabrication. The pattern of mask156 may include various image structures, for example, clear areas,opaque areas or even phase shifting areas.

As one skilled in the art will know, a physical "binary" mask is onewhere the mask is clear in some areas and opaque in other areas. Thecombination of clear areas and opaque areas in conjunction with incidentlight, image the mask design in the light-sensitive material. A mask-mayalso include a phase shifting region. Phase shifting is where there isincluded, in or on the mask (i.e., in a different plane than the rest ofthe mask), a means for altering the phase of light passing therethrough.This creates a phase difference that is useful for imaging smallerfeatures. A further discussion of phase shifting can be found in U.S.Pat. No. 5,300,786 issued to Brunner et al. and assigned to IBM, whichis herein incorporated by reference in its entirety.

As used herein, the term "light" refers to light used inphotolithography. The terms "light" and "photolithography" in thespecification need not be restricted to visible light, but may alsoembrace other forms of radiation and lithography. For example, energysupplied by lasers, photons, ion beams, electron beams or x-rays are allincluded within the term "light". Accordingly, the term "mask" is meantto include not only a physical structure, but also a digitized imageused in, for example, electron beam and ion beam lithography systems.Light passing through mask 156 intersects a lens 158, which may be, forexample, a reduction lens for focusing the mask pattern onto the objectfor imaging 160, in this case a semiconductor wafer. The object forimaging 160 is held in position by, for example, a vacuum hold devicewhich is part of and controlled by an accurate X, Y, Z locationcontroller or stepper 162.

One generalized embodiment of a print bias measuring tool 152 pursuantto the present invention is depicted in FIG. 3. Print bias measuringtool 152 is coupled to projection imaging system 150. It will beunderstood that print bias measuring tool 152 may be automated or may beoperated by a human operator. The main purpose of print bias measuringtool 152 is to detect an isolated edge within a print bias target imagedin the object for imaging 160, and determine a print bias therefor withrespect to the design image of mask 156, which computer 166 holds inmemory. Print bias measuring tool 152 comprises isolated edge detector164, computer 166 and input/output device(s) 168. Isolated edge detector164 detects, directly or indirectly, isolated edges and computer 166compares the positioning of the isolated edges detected to that of thedesign image of mask 156 stored in memory (not shown). Such informationcan be displayed or stored through appropriate I/O devices 168 foroperator information which may be acted on thereby, or employed as anautomated feedback control signal (via line 170) to automatedly controlprojection imaging system 150 to improve the imaging of the object 160.For example, for the basic print bias method of the present invention,illumination source 154 may be altered based on the results. As anotherexample, the focus setting of lens 158 may be altered where performanceof the method of determining aberrations of the present inventionidentifies an aberration. As still a further example, lens 158 focusand/or object positioning via stepper 162 may be altered based on theresults of performing the overlay method of the present invention.Computer 166 may comprise, for example, local memory, a centralprocessing unit and an arithmetic and logic unit. I/O device(s) 168 maycomprise, for example, a monitor, a mouse, a keyboard and appropriatestorage and retrieval systems. Whatever print bias measurement system isused, it is important that it have the ability to establish a consistentfocus; that is, the focusing subsystem should be of good quality. Itwill be understood that various measurement systems are capable ofaccomplishing the task of print bias measuring tool 152, such as, forexample, a scanning electron microscope, a scanning confocal microscope,various optical measurement systems (using the wavelength of lightinstead of electrons to measure) or the alignment system of aphotolithography tool.

FIG. 4 depicts print bias target 16 of FIG. 2 under conditions ofunderexposure by the lithography system. As one skilled in the art willknow, underexposure of a positive resist causes larger features to beimaged than called for by the mask. Underexposure can be seen in target16 by inner box 30 being off-center with respect to outer box 28; morespecifically, inner box 30 appearing to have moved toward the lower lefthand corner of outer box 28. Due to underexposure, regions 20 and 24 oftarget 16 are larger than intended in the design image (see FIG. 5). Inan actual measurement system, underexposure of target 16 is indicated bydetecting isolated edge 32 and determining that it is not in the correctposition according to the design image. In the case of FIG. 4, anegative print bias value is preferably returned.

FIG. 5 depicts print bias target 16 at what will be termed herein as"best exposure", meaning that inner box 30 is centered within outer box28. For the best exposure situation, a print bias value of zero (0) foreach edge type (here, horizontal and vertical edges) would preferably bereturned, and is the most faithful replication of the print bias target.

FIG. 6 depicts a situation where print bias target 16 has beenoverexposed by the lithography system. Overexposure is characterized byinner box 30 being off center within outer box 28, and morespecifically, inner box 30 appearing closer to the top right corner ofouter box 28 as compared to the best exposure situation of FIG. 5.

FIG. 7 is a flow diagram of a method for determining print bias withrespect to a design image in a lithographic system. The method of FIG. 7begins by imaging a print bias target in a single layer of a lightsensitive material (STEP 34, "IMAGE PRINT BIAS TARGET IN SINGLE LAYER").As with print bias target 16, the exemplary print bias target of thepresent method includes a plurality of target regions (for example,region 20 in print bias target 16), each target region having an imagestructure different from that of all other target regions immediatelyadjacent thereto. The interspersing of different image structure targetregions within print bias target 16 improves the detection of isolatededges used to determine print bias by providing isolated edges borderingdifferent regions. After the print bias target has been imaged, aposition of an isolated edge in the imaged print bias target relative toa corresponding isolated edge in the design image of the mask isdetermined (STEP 36, "DETERMINE POSITION OF ISOLATED EDGE RELATIVE TODESIGN IMAGE").

The step of imaging a print bias target according to the presentinvention (STEP 34) includes providing a mask having a print bias targetdesign. The imaging step also includes providing a substrate with alight-sensitive material coated thereon, in which a print bias targetmay be imaged. This imaging is accomplished by shining light through themask or selectively shining light according to the image design toexpose selected portions of the light-sensitive material to light. Forexample, assuming a positive resist and an optical imaging system,regions 18, 22 and 26 in print bias target 16 of FIG. 2 were all exposedto light that has passed through a physical mask having the print biastarget design thereon. After exposure, selected portions of thelight-sensitive material are removed. In the context of print biastarget 16, regions 18, 22 and 26 indicate places where the positiveresist was exposed to light and subsequently removed, for example, byusing a solvent or by etching.

The step of determining a position of an isolated edge relative to thedesign image comprises the measurement device searching the imaged printbias target for a given isolated edge, for example, isolated edge 32 inFIG. 4. The measurement device compares the detected isolated edge witha corresponding edge in the design image of the mask to determinewhether the isolated edge is placed properly, and if not, to what extentit is misplaced. With respect to exemplary print bias measurement tool152, the comparison is accomplished through computer 166 programmedappropriately. In the context of the present embodiment, a print biasvalue will be returned by the measurement device indicative ofunderexposure, overexposure or proper exposure. For example, in thepresent embodiment, a negative print bias value indicates anunderexposed condition, a positive print bias value indicates anoverexposed condition and a print bias value of zero in this simplifiedembodiment indicates the best exposure condition. However, it will beunderstood that best exposure may be other than zero in practice, forexample, if overexposure to obtain smaller line width is necessary.

After determining print bias, the information can be used to adjust thelithographic system in an attempt to remove or decrease the degree ofprint bias. Where the print bias information is fed back to thelithography system for improving same, the print bias targets would be,in the case of semiconductor chips, included on a product wafer, forexample, in the kerf section thereof. However, the print bias targetcould also be used on sacrificial wafers to "calibrate" the lithographicsystem prior to processing of product wafers.

FIG. 8 depicts print bias target 16 along with actual and ideal profilesthereof. Ideal profile 37 and actual profile 39 are taken along line 41.Similarly, ideal profile 43 and actual profile 45 are taken along line47. FIG. 8 shows that each isolated edge, in practice, will have sometype of a slope associated therewith. Ideally, resist portion 38corresponding to section 20 in print bias target 16 creates a 90° angleon either side thereof with respect to substrate 40. However, inpractice a profile of print bias target 16 shows that section 20,corresponding to profile section 42, has a slope associated with each ofthe isolated edges 44 and 46 thereof. The slope information is usefulfor biasing the next process step, for example, etching trenches insilicon. As another example, the slope information may help determinewhether an unusually high print bias was due to de-focus or exposure.

FIG. 9 depicts an enlarged version of profile section 42 from FIG. 8.FIG. 9 also illustrates, in conjunction with FIG. 10 (discussedsubsequently), the effect of the measurement systems' depth of focus onprint bias determination. As shown in FIG. 9, the top edge of profilesection 42 is first seen at Z focus position Z1. As the Z focus of themeasurement system is changed from Z1 to Z2, the measurement system willinterpret different portions of slope 48 to be the isolated edge. Thiswill, of course, produce different print bias information depending onthe Z focus of the measuring system.

FIG. 10 plots, along line 50, average print bias versus Z focus for Xedge slope 52 in FIG. 8, and along line 54 plots average print biasversus Z focus for Y edge slope 48 in FIG. 9. Each data point along eachof lines 50 and 54 is an average of three print bias values for the sameisolated edge at a given Z focus setting. Preferably, at least threeprint bias determinations are made at each focus setting in order to geta sense of the repeatability of the measurement at that setting. Inaddition, FIG. 10 plots variations in print bias determinations at agiven Z focus for both X edge slope 52 and Y edge slope 48. Morespecifically, each bar graph portion of FIG. 10 is three times thestandard deviation of the print bias values associated with thecorresponding focus setting. As can be seen from FIG. 10, print biasinformation obtained at the periphery of the Z focus settings for themeasurement system is inherently unreliable. However, from about -1.5 onthe Z focus scale to about 0.0 on the Z focus scale, lines 50 and 54approach linearity to the point where a best-fit line can be determined,as well as an associated slope therefor.

Before the slope of an isolated edge profile is determined, severalother parameters will be examined. The measurement system's totaldepth-of-focus is calculated by:

    depth-of-focus=w/na,

where

w measurement system nominal wavelength of light used duringmeasurement, and

na numerical aperture of the measurement system lens.

In the present embodiment, assume a nominal wavelength of 0.5 micronsand a numerical aperture of 0.5. As indicated previously, not all theprint bias measurements taken along the edge profile are reliable (seethe X and Y-three sigma bars in FIG. 10). However, one can expect arange of Z focus settings where a best-fit line can be drawn through thecorresponding print bias values. A range of expected low three-sigmameasurements can be determined by adding the thickness of the layer oflight-sensitive material used together with the measurement system'sdepth of focus. Assume, in the present embodiment, a range of 2 microns,i.e., a thickness of 1 micron and a depth of focus of 1 micron. To findthe slope of the isolated edge profile, a best fit line is drawn for theprint bias data points over the range of low three-sigma measurements.The best-fit line may be drawn by conventional methods, such asregression analysis. The result of the best-fit line equation, in theform of y=mx+b provides an X coefficient (i.e., m), which, if divided bytwo, provides the average edge slope of a single edge. The angle for thebest-fit line in degrees is found by:

    slope=arc cos ((m* -0.001/2)=88.3 degrees.

Note that in the present exemplary embodiment, the X coefficient of thebest-fit line is -0.060 nm/microns. Also note that the value -0.001converts nanometers to microns and provides a positive slope value indegrees.

Finding the profile slope in accordance with the present inventionprevents having to use destructive methods currently used to obtain thesame information. For example, one way to examine the profile is bybreaking the substrate and using a scanning electron microscope to get apicture of the profile; then, using a ruler and protractor, the slopemay be determined from the picture. However, this common method is notvery accurate and, as indicated previously, is destructive to thesubstrate involved. Finding the profile slope according to the presentinvention is non-destructive and can be done with an accuracy of up to±0.1°.

FIG. 11 is a flow diagram of a method for determining a slope of animaging profile for an image created in a lithographic system accordingto the present invention. The method begins by imaging a print biastarget in a single layer of light-sensitive material (STEP 70, "IMAGEPRINT BIAS TARGET IN SINGLE LAYER"). After imaging the print biastarget, the measurement system Z focus is initialized, for example, to alow setting (STEP 72, "INITIALIZE Z FOCUS"). The print bias for ahorizontal isolated edge is then determined a number of times, forexample, three, and an average print bias is obtained (STEP 74,"DETERMINE AVERAGE PRINT BIAS FOR HORIZONTAL ISOLATED EDGE"). Thestandard deviation of the horizontal isolated edge print bias values isthen found (STEP 75, "DETERMINE STANDARD DEVIATION"). The print bias fora vertical isolated edge is also determined a number of times and anaverage found (STEP 76, "DETERMINE AVERAGE PRINT BIAS FOR VERTICALISOLATED EDGE"). The standard deviation of the vertical isolated edgeprint bias values is then found (STEP 77, "DETERMINE STANDARDDEVIATION").

After determining average print bias for both the horizontal andvertical isolated edges, the system Z focus is increased (STEP 78,"INCREASE Z FOCUS").

After increasing the Z focus, an inquiry is made as to whether the Zfocus is beyond the print bias target bottom (INQUIRY 80, "Z FOCUSBEYOND PRINT BIAS TARGET BOTTOM?"). If the system Z focus is not beyondthe print bias target bottom, then the method returns to step 74. If thesystem Z focus is beyond the print bias target bottom, a plot is made ofthe average print bias values obtained (STEP 82, "PLOT AVERAGE PRINTBIAS VALUES"). After plotting the print bias values, best fit lines forthe lowest standard deviation print bias values within a predeterminedrange of Z focus settings for each of the horizontal and verticalisolated edge print bias data are derived (STEP 84, "DERIVE BEST-FITLINES"). The derivation may be, for example, a mathematical derivation,for example, using regression analysis, or it may be physically derivedby "eye-balling" the plotted print bias values. Finally, after derivingthe best fit lines, the slope thereof is determined (STEP 86, "DETERMINESLOPE OF BEST-FIT LINES").

The major factors affecting the quality of the print bias informationreturned by the measurement tool are exposure and focus, with exposurebeing affected by either the intensity of the incident light or theamount of time a particular area of light-sensitive material is exposed.A print bias target according to the present invention can be used inconjunction with a multivariant test to determine an appropriate focusand/or exposure for a lithographic system.

FIG. 12 depicts a matrix 56 comprised of print bias target 16 imaged atvarious exposure and focus settings. Matrix 56 is created by imaging aprint bias target mask design at a given exposure and focus setting,then microstepping to a location adjacent to the previously imaged printbias target with a slightly different exposure and/or focus setting andre-imaging the same target design. This procedure is followed until amatrix of a desired size is created, from which valuable information canbe extracted. It will be understood that matrix 56 is shown as a simplematrix comprising nine print bias targets for ease of discussion only.In practice, such a matrix may include a considerably greater number oftargets. It will also be understood that matrix 56 would not usually beimaged on a semiconductor product wafer in a lithographic system using aphysical mask, since successive imaging of the print bias target mayharm the product. However, such a matrix may be included on a productwafer where a non-physical mask is used, for example, E-beam or ion beamimaging systems.

FIG. 13 is a three-dimensional plot of print bias versus focus andexposure obtained from a print bias target matrix similar to that shownin FIG. 12, but of a larger size. Focus setting is plotted along the Xaxis of plot 102, exposure setting is plotted along the Y axis and printbias is plotted along the Z axis. A silhouette of the three-dimensionalplot against the X, Z plane 104 provides information for determiningbest focus for the lithographic system used to image the print biastarget matrix corresponding thereto. Similarly, a silhouette of thethree-dimensional plot against the Y, Z plane 106 provides exposurescurves for determining best exposure. One way to determine best exposureusing the silhouette in the X, Z plane 104 will subsequently bedescribed. Another way to determine best exposure using the silhouettein the Y, Z plane 106 can be found in an article by H. Ohtsuk, K. Abeand Y. Itoh, entitled "Quantitative Evaluation Method At Conjugate Pointfor Practical Resolution of Wafer Stepper", appearing in SPIE Vol. 1088Optical/Laser Microlithography II (1989) at page 124-132, which isherein incorporated by reference in its entirety.

A preferred method for determining best focus for a lithographic systemwill now be described in detail with respect to FIG. 13. After imaging aprint bias target matrix and creating a three-dimensional plot, such asthat shown in FIG. 13, the determination of best focus centers on thecurves created by a silhouette of the three-dimensional plot in the X, Zplane 104. Prior to determining best focus, the print bias data obtainedfrom the print bias matrix is fitted to a mathematical modelrepresenting the three-dimensional plot of FIG. 13. For example, thelines shown connecting data points in the three-dimensional plot of FIG.13 represent the model that has been derived to fit those data points.Note that the print bias scale in FIG. 13 starts at line 110, ispositive above line 110 and negative below line 110. The focus curvesachieved from a silhouette of the three-dimensional plot, appearing inplane 104 of FIG. 13, include curve 105 at a minimum exposure, curve 107at a maximum exposure, curve 109 at an exposure between the two extremesand a curve referred to as an "isofocal point" 111. The isofocal point111 is a focus curve approaching or achieving linearity. Note that thederivation of the mathematical model is based in part on the type ofequipment used, the particular process involved and the type oflight-sensitive material being used. In short, the mathematical modelmay be derived using conventional derivation techniques, and will varyfrom application to application. Also note that the best fitting of theprint bias data to the mathematical model may be done using conventionalmethods, such as regression. It will be understood that although curves105, 107 and 109 are identified herein for determining best focus, onecould use more than three of the focus curves; the use of three curvesrepresents the preferred minimum number of curves. A point on each ofthe identified curves 105, 107 and 109 is identified which is closest toa desired print bias value. For example, assume that line 110 representsa desired print bias value. Thus, a point on curves 107 and 109representing a maximum for those curves would be identified, and a pointon curve 105 representing the minimum thereof would also be identified.The identified points on curves 105, 107 and 109 are then best fit to astraight line. Where the best-fit line intersects the isofocal point 111is the point that determines the best focus. Thus, a vertical line fromthe intersection point is drawn to the focus axis to determine bestfocus. One way to determine best exposure using the focus curves is toidentify the isofocal point and determine which exposure setting causedsame. Note that the present method of determining best focus from athree-dimensional plot of print bias versus focus and exposure is mostreliable when the lithographic system is operating at five times itsresolution limit or higher, since at such resolution the behavior ofprint bias is similar to that of line width.

FIGS. 14 and 15 comprise a flow diagram of a method for determining bestfocus for a lithographic system according to the present invention. Themethod begins by imaging a print bias target matrix (STEP 88, "IMAGEPRINT BIAS TARGET MATRIX"). Step 88 may be performed by initializing theexposure and focus settings for the lithographic system, imaging a printbias target, determining whether the matrix is completed, and if not,microstepping to another location and altering the exposure and/or focusand imaging another print bias target. The process is repeated until theprint bias target matrix is completed. After the print bias targetmatrix is imaged, the print bias of the same isolated edge in eachtarget of the matrix is determined (STEP 90, "DETERMINE PRINT BIAS OFSAME ISOLATED EDGE IN EACH TARGET"). After determining print bias, athree-dimensional plot is created for print bias against focus andexposure (STEP 92, "CREATE 3-D PLOT OF PRINT BIAS AGAINST FOCUS ANDEXPOSURE"). Based on the equipment used, the particular process, thetype of light-sensitive material being used and various other factors, amathematical model is derived for the relevant imaging process in termsof print bias, focus and exposure (STEP 93, "DERIVE MATHEMATICALMODEL"). The print bias data is then best fit to the mathematical model,shown in FIG. 13 as the "net" connecting the data points (STEP 94, "BESTFIT PRINT BIAS DATA TO MATHEMATICAL MODEL"). After the print bias datais best fit to the mathematical model, the focus curves are obtained bycreating a silhouette of the best fit plot in the print bias versusfocus plane 104 (STEP 95, "OBTAIN FOCUS CURVES FROM SILHOUETTE OFBEST-FIT PLOT IN PRINT BIAS VS. FOCUS PLANE"). Focus curves for high,medium and low exposures are then identified (STEP 96, "IDENTIFY FOCUSCURVES FOR HIGH, MED AND LOW EXPOSURES"). In addition, the isofocalpoint is also identified (STEP 97, "IDENTIFY ISOFOCAL POINT"). A pointon each of the high, medium and low focus curves that is closest to adesired print bias is then identified (STEP 98, "IDENTIFY POINT ON HIGH,MED, LOW FOCUS CURVES CLOSEST TO DESIRED PRINT BIAS"). The closestpoints on the high, medium and low focus curves are then best fit to astraight line (STEP 99, "BEST FIT LINE TO CLOSEST POINTS"). The pointwhere the best fit line and the isofocal point intersect is then used todetermine best focus by identifying where a vertical line from theintersection point intersects the focus axis (STEP 100, "IDENTIFY BESTFOCUS POINT WHERE BEST FIT LINE AND ISOFOCAL POINT INTERSECT"). It willbe understood that the steps of the flow diagram of FIGS. 14 and 15beyond Step 90 may be determined by an appropriately programmed computerassociated with the print bias measuring device. Thus, an operator ofsuch a print bias measuring device may not actually see thethree-dimensional plot, the silhouette or the best fit line.

Print bias information obtained from a print bias target matrixaccording to the present invention is useful not only for determiningbest exposure and/or best focus, but also to identify aberrations in theprint bias target caused by the lithographic system and/or theenvironment. FIG. 16 is a plot of print bias versus focus setting atseveral different exposures for both a horizontal and a verticalisolated edge according to matrix 56 in FIG. 12. FIG. 16 depicts thecase where no aberrations are present in the print bias target, shown bythe curves being coincident for both the horizontal and verticalisolated edge of interest.

FIG. 17 depicts the case where an lithographic astigmatism is present inthe lithographic system, shown by displacement along the focus axis ofthe vertical edge (dotted lines) curves from the horizontal (solidlines) edge curves. Lithographic astigmatism in the present embodimentis where the best focus for each of the vertical and horizontal isolatededges does not coincide. Where an lithographic astigmatism is found, itcan be corrected, at least to some extent, by altering the lithographicsystem focus setting, based on the difference between the best focus forthe horizontal and vertical edge curves.

FIG. 18 depicts a situation of lithographic coma; that is, where thehorizontal and vertical isolated edges tend to become different sizesfrom one another with variations in exposure. In FIG. 18, this is shownby less than all the curves for the vertical isolated edge (dottedlines) and the curves for the horizontal isolated edge (solid lines)being displaced vertically from one another. Where lithographic coma isfound in a lithographic system, it can be corrected, at least to someextent, by adjustments to the lithographic system, such as the lens inFIG. 3.

FIG. 19 depicts the situation of vibration in the lithographic system;that is, micromovements occurring during imaging, causing smearing ofthe projected image during imaging which causes displacement of thehorizontal and vertical isolated edges. Note also that the slope of theresulting image profile will degrade. In FIG. 18, vibration isidentified by all the vertical isolated edge curves (dotted lines) beingdisplaced horizontally from the horizontal isolated edge curves (solidlines). Vibration in the lithographic system can be corrected for bydetermining the cause of the vibrations and controlling same. Once it isdetermined that one or more aberrations exist, the lithographic systemand/or the environment could be adjusted, using the aberrationinformation in a feedback manner.

The print bias target of the present invention is useful not only forproviding print bias information, leading to various other informationregarding the lithography system, but also for overlay. FIG. 20 depictsan overlay target 122 according to the present invention. Overlay target122 comprises an overlay box 124 at a lower level on top of which isimaged print bias target 16 at a second lithographic level. Utilizingthe print bias target of the present invention along with the lowerlevel overlay box provides the necessary overlay information withoutgoing to a different tool than required for the print bias information,saving time and money. In general, the results of the present overlayprocedure provide data for controlling the imaging process. Thisprocedure is based on the overlay procedure already existing today andwhich was described with reference to FIG. 1.

In contrast to the current overlay rod the overlay method of the presentinvention focuses on comparing the center of the lower level image withthat of the average center of the upper level print bias target. Asexposure and/or focus vary, the position of the outer isolated edge 154of print bias target 16 may change relative to inner isolated edge 152,however, the mid-point between isolated edges 154 and 152 with respectto outer box isolated edge 150 does not change, since edges 152 and 154are isolated edges of differing image structure and therefore counteracteach other. Note that this is true so long as the print bias target as awhole is within outer box 124.

Isolated edge detection windows, e.g., detection window 132, are theareas in overlay target 122 where the overlay measurement systemsearches for an isolated edge. An example of the determination ofoverlay for overlay target 122 will now be given. The example will focuson determining the X overlay coordinate with it being understood thatthe Y overlay coordinate is determined in a similar fashion. The Xcoordinate center of lower level image 124 is determined by adding themeasurement for the isolated edge in detection window 132 to themeasurement for the isolated edge in detection window 127 and dividingthe sum by 2. The average center of print bias target 16 is found byfirst finding the midpoint between the isolated edges within detectionwindows 123 and 125. This is, of course, merely an average of themeasurements for those isolated edges. Similarly, the midpoint is foundbetween isolated edges found within detection windows 129 and 131. Oncethe midpoints are found, they are considered to be the X coordinateedges of the upper level image. As such, the midpoints are addedtogether and divided by 2 to find the X coordinate of the averagecenters of print bias target 16. The X overlay coordinate is then foundby subtracting the X coordinate for the average center of print biastarget 16 from the X coordinate center of lower level image 124.

In addition to imaging print bias target 16 over a larger lower levelimage to obtain the overlay target, the lower level image could bephysically smaller than the print bias target image. Note, however, thatone should avoid imaging the edges of the print bias target in closeproximity to the edges of the lower-level image. One should also striveto keep the print bias target within it topography. FIG. 22 depictsoverlay target 133 comprising print bias target 16 which has been imagedon top of lower level image 134, the outline of which is visible in FIG.22. Overlay is determined in the same manner as described with respectto FIG. 19. In this case, overlay target 16 is centered on top of lowerlevel image 134; therefore, the overlay coordinates would be (0,0). FIG.21 depicts a situation where print bias target 16 is not centered on topof lower level image 134. Lower level image 134 appears to be up and tothe right of the center of print bias target 16, producing negativeoverlay coordinates. The opposite situation (positive overlaycoordinates) is depicted in FIG. 23.

FIG. 24 is a flow diagram of a method for determining overlay in alithographic system according to the present invention. The methodbegins by imaging the overlay target (STEP 141, "IMAGE OVERLAY TARGET").Imaging the overlay target comprises imaging the lower level image, andthen imaging the print bias target on top thereof in accordance with thepresent invention. After imaging the overlay target, the isolated edgesof the upper and lower images are located (STEP 142, "LOCATE ISOLATEDEDGES OF UPPER AND LOWER IMAGES"). With reference to FIG. 20, STEP 142is accomplished by searching for the isolated edges in the isolated edgedetection windows (e.g., window 132). After locating the isolated edgesof the overlay target, the X and Y coordinates for the lower level imagecenter are determined (STEP 143, "FIND X, Y COORDINATES FOR LOWER LEVELIMAGE CENTER"). The X, Y coordinate for the average center of the upperlevel image, i.e., the print bias target, is also determined (STEP 144,"FIND X, Y COORDINATES FOR AVERAGE CENTER OF UPPER LEVEL IMAGE"). Afterthe X and Y coordinates of the lower level image center and upper levelimage average center are determined, the difference between thecorresponding coordinates is determined in order to obtain the overlaycoordinate result (STEP 146, "DETERMINE DIFFERENCE BETWEEN X COORDINATESFOR X OVERLAY COORDINATE"; and STEP 148, "DETERMINE DIFFERENCE BETWEEN YCOORDINATES FOR Y OVERLAY COORDINATE"). From the results of the overlaydetermination, the registration of the print bias target with respect tothe lower level image is found. The resultant overlay coordinate maythen be compared to tolerances specified in a given situation todetermine whether the overlay is within acceptable parameters.

While several aspects of the present invention have been described anddepicted herein, alternative aspects may be effected by those skilled inthe art to accomplish the same objectives. Accordingly, it is intendedby the appended claims to cover all such alternative aspects as fallwithin the true spirit and scope of the invention.

We claim:
 1. A mask for determining print bias in a lithographic system,the mask comprising a first plurality of areas substantially clear tolight of a given type and a second plurality of areas substantiallyopaque to light of the given type, wherein the first plurality of areasand the second plurality of areas together comprise a mask design, themask design a comprising an inner binary geometric shape of a givengeometric type and an outer binary geometric shape of the givengeometric type encompassing and concentric with the inner binarygeometric shape, each binary geometric shape divided into equal halvesalong the same axis, the inner binary geometric shape comprising one ofthe first plurality of areas as a first half thereof and one of thesecond plurality of areas as a second half thereof, and the outer binarygeometric shape comprising one of the second plurality of areas as afirst half thereof adjacent the first half of the inner binary geometricshape and one of the second plurality of areas as a second half thereofadjacent the second half of the inner binary geometric shape.
 2. Themask of claim 1, wherein said mask comprises a physical mask.
 3. Themask of claim 1 further comprising a phase shifting region.
 4. The maskof claim 1 wherein said given geometric type comprises a square.
 5. Themask of claim 1, wherein the given type of light comprises visiblelight, wherein the first plurality of areas are substantially clear tovisible light, and wherein the second plurality of areas aresubstantially opaque to visible light.